1. Field of the Invention
The present invention relates to a photographic apparatus and method for sensing insufficient flash illumination. More particularly, this invention relates to a camera capable of sensing insufficient flash illumination and a control method by which a warning is given when the illumination is insufficient
2. Description of the Related Art
Conventionally, an electronic flash (a.k.a. "strobo", hereinafter referred to as "flash") unit is mounted to a camera and used to supplement object illumination when photographs are taken at night or indoors. The flash unit is usually mounted in the camera for middle-sized and small-sized cameras. Flash units may be classified as either manual or automatic. In a manual flash unit system, the flash emits a predetermined amount of illumination regardless of the distance to the object. In an automatic flash system, the amount of flash illumination emitted is controlled depending on the distance between the camera and the object.
Both types of flash systems operate by charging a power source, such as a capacitor, which is triggered to discharge at the time the photograph is taken. A light emitting lamp is usually used to emit an amount of illumination proportional to the charged voltage level of the capacitor. Accordingly, the charged state of the capacitor should provide sufficient discharge voltage to drive the lamp in order to sufficiently illuminate the object.
Conventionally, the following formula (1) is used to determine the proper amount of illumination for a suitable exposure after the distance from the object is determined by an automatic distance measuring circuit incorporated in the camera. ##EQU1##
Since it is usually difficult to distinguish an object from the background, oftentimes the distance to the object determined by the camera will be for the background instead of the object. When this happens, more illumination is provided than necessary resulting in an overexposed photograph.
Therefore, in the related art, the user can obtain sufficient exposure by increasing the brightness of the lens by reducing the aperture size of a diaphragm. Additionally, a portion of the charge on the capacitor may be shunted from the lamp to reduce illumination or the light emitted from the lamp can be blocked in some manner to reduce the amount of light directed toward the object.
However, since the above-mentioned methods are limited to correcting exposure when the object is close, objects that are more distant would not receive sufficient illumination. For example, if the illumination of the flash (i.e., guide number of the flash) is 12, the maximum brightness (i.e., maximum F number) of the lens is 3.5, and the sensitivity of the film is 100, the maximum limit for sufficient exposure occurs at a distance 3.4 m according to equation (1). Accordingly, if the distance to the object is 7 m, and the maximum brightness of the lens is still 3.5, then the amount of illumination required becomes 24. Thus, it is possible to take a suitably exposed photograph with the same lens setting only if the object illumination is doubled.
When a photograph is taken without recognizing the above-mentioned problems, the amount of light is not sufficient and film is wasted. An example of a technique for disabling the shutter until the flash is sufficiently charged is suggested in Korean Utility Model Publication No. 91-9033, entitled "AN AUTOMATIC CONTROL CIRCUIT FOR A FLASH". Upon detection of a low light signal, the control circuit will wait for a predetermined time for the flash unit to charge before opening the shutter. However, this can be an inconvenience since the shutter operates regardless of the intention of the user. Thus, the user cannot take a photograph at a chosen moment in time.